A student carries out a reaction to produce iron oxide. The theoretical yield of iron oxide is 8.0 g, but the student only obtains 6.0 g of iron oxide. Explain why the actual yield is less than the theoretical yield, and describe what is meant by percentage yield.

Edexcel GCSE Chemistry (1CH0) — 5.2 Quantitative analysis (Chem only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In industrial and laboratory chemistry, reactions rarely produce the maximum possible amount of product. Chemists use percentage yield to compare how much product was actually obtained with how much could theoretically have been made.

Model answer (4 marks)

The actual yield is less than the theoretical yield because the reaction may be incomplete, so not all reactants are converted into product.
The actual yield may also be reduced by side reactions that produce unwanted by‑products.
Loss of product during transfer, filtering or evaporation can further decrease the actual yield.
Percentage yield is the percentage ratio of the actual yield to the theoretical yield, calculated as (actual yield ÷ theoretical yield) × 100.

Examiner tips

  • Use the exact phrase ‘percentage yield is the percentage ratio of the actual yield to the theoretical yield’
  • Show the calculation formula (actual ÷ theoretical × 100)
  • Mention all three common reasons for lower actual yield

Common mistakes

  • Confusing theoretical yield with stoichiometric yield
  • Omitting the calculation formula for percentage yield
  • Failing to list all three reasons for reduced actual yield

Mark scheme (4 marks)

  1. The actual yield is less than the theoretical yield because the reaction may be incomplete / not all reactants are converted to products
  2. The actual yield may be reduced due to side reactions producing unwanted by-products
  3. The actual yield may be reduced due to loss of product during transfer between containers / filtering / evaporating
  4. Percentage yield is the percentage ratio of the actual yield compared with the theoretical yield (actual yield ÷ theoretical yield × 100)

Key terms in this question

actual yield · theoretical yield · percentage yield

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